tcg/loongarch64: Fix tcg_out_movi vs some pcrel pointers

tcg/loongarch64: Fix tcg_out_movi vs some pcrel pointers

Simplify the logic for two-part, 32-bit pc-relative addresses.
Rather than assume all such fit in int32_t, do some arithmetic
and assert a result, do some arithmetic first and then check
to see if the pieces are in range.

Cc: qemu-stable@nongnu.org
Fixes: dacc51720db ("tcg/loongarch64: Implement tcg_out_mov and tcg_out_movi")
Reviewed-by: Song Gao <gaosong@loongson.cn>
Reported-by: Song Gao <gaosong@loongson.cn>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>